Reuse TemplateList class when deleting a folder

Part of moving "get_template_folders" et al. into TemplateList so we
can cache it more effectively. This is slightly less efficient as
iterating a TemplateList will instantiate an object for each item
in the folder; but the difference is minimal.

Note that:

- The default template_type for TemplateList is "all".
- We need to pass realistic template "JSON" in the test now.

app/models/service.py

@@ -650,12 +650,6 @@ class Service(JSONModel, SortByNameMixin):
             template,
         ]
 
-    def get_template_folders_and_templates(self, template_type, template_folder_id):
-        return (
-            self.get_templates(template_type, template_folder_id)
-            + self.get_template_folders(template_type, template_folder_id)
-        )
-
     @property
     def count_of_templates_and_folders(self):
         return len(self.all_templates + self.all_template_folders)